The best alternatives to Fast.io are Firefox Send, Shelf, and Envelop. If these 3 options don't work for you, we've listed a few more alternatives below.
ToffeeShare is a private and secure way of sending files via your browser. Your files are never stored in 'The Cloud' but are transferred directly (peer to peer) to the receiver. There's no file size limit and ...
ShareFiles is an hassle-free way to share any file/folder you want with a link. Links will automatically expire after 24 hours. There is no registration or payments necessary to use this product now.
DataSkimmer makes it easy to share your files the way you want to: on brand, easily skim-able, secure, and able to be updated without re-sending anything.